1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device using an organic semiconductor.
2. Description of the Related Art
Nowadays, the active functionality of an organic semiconductor attracts attention, and a semiconductor element using an organic semiconductor is widely studied. In terms of the capability of material design, functionality added to an organic semiconductor has infinite possibilities, and an advantage due to utilizing an organic material as a semiconductor is immeasurable.
However, a semiconductor element using an organic semiconductor generally has low carrier mobility compared with a semiconductor element using an inorganic semiconductor; therefore, further enhancement of carrier mobility of an organic semiconductor element is one of the tasks of applying an organic semiconductor to a semiconductor element.
The basic structure unit of an organic semiconductor is a molecule, and the aggregation structure of the organic semiconductor is formed due mainly to intermolecular force. Hence, the film quality (amorphous, poly-crystal, or single crystal), molecular sequence, or molecular orientation of an organic semiconductor depends on a molecule-specific shape (spherical shape, planar shape, straight chain shape, or the like), a film formation method, or a substrate.
It is well known that molecules are oriented vertically or parallel to a substrate in accordance with a molecular shape under the restraint of, especially, a substrate in the case of forming a thin film using an organic semiconductor.
In the case of π-conjugated system straight chain molecules such as pentacene, carriers are easy to flow since π electron interaction is strong in a stacking direction in which molecular aspects are face to face with each other. Therefore, in the case where π-conjugated system straight chain molecules are oriented vertically, carriers flow in a direction perpendicular to a long axis of a molecule. In the case of employing a horizontal structure such as an organic field effect transistor, it is understood that carrier mobility is improved when the molecular orientation of pentacene is oriented vertically.
Carrier mobility of a semiconductor element using an organic semiconductor can be improved by controlling the molecular sequence of a semiconductor element and making molecules be oriented so that carrier mobility is improved in an organic semiconductor layer (so that the overlap of π-π between molecules is generated in a direction which carriers flow between a source electrode and a drain electrode) (for example, refer to Patent Document 1: Japanese Patent Laid-Open No. H9-232589)
A rubbing method can be given as a method for controlling molecular orientation of an organic semiconductor. A rubbing method is a method for obtaining orientation capacity for the molecules of an organic semiconductor layer by rubbing an organic semiconductor layer directly or an orientation film provided over a substrate in one direction with a cloth having long hairs or the like. Accordingly, orientation treatment of a large-sized substrate can be carried out in a short period of time, and uniform in-plane orientation (uniaxial orientation) of molecules to a whole surface can be obtained.
Desired molecular orientation can be formed and carrier mobility can be improved by using an organic semiconductor layer in which molecules are oriented by a rubbing method for a semiconductor element. However, some problems occur in the case of performing rubbing treatment for an organic semiconductor layer to be applied to a semiconductor element.
As one of the problems, an organic semiconductor layer which forms a channel is physically damaged by rubbing the organic semiconductor layer which forms a channel due to the softness which is specific to an organic semiconductor material, which leads to a problem that OFF current increases or gate leak current may occur due to the breakdown of a gate insulating film. Secondly, in the case of rubbing a gate insulating film to orient molecules in an organic semiconductor layer stacking over the gate insulating film, a minute groove structure is formed by performing rubbing treatment over the surface of the gate insulating film; therefore, a planar surface cannot be obtained in the interface between an organic semiconductor which forms a channel and the gate insulating film, which leads to a problem that carrier mobility is difficult to be sufficiently improved.